This invention relates to an exposure apparatus which carries a substrate such as a reticle or a photomask and projects a circuit pattern formed on the substrate upon a wafer, whereby the wafer is exposed to light by utilizing a projection lens or a reflection mirror.
For example, U.S. Pat. Nos. 4,153,371 and 4,362,389 disclose a reduction projection exposure apparatus in which when a circuit pattern formed on a substrate such as a reticle or a photomask is transcribed upon a wafer with a reduction ratio and then chips are exposed to light one by one, a pellicle (a member consisting of a metal frame and a transparent thin film of nitrocellulose applied to the metal frame) adapted to prevent deposition of foreign particles on foreign matter is installed on the substrate in order to prevent the substrate from being deposited with foreign particles. In principle, the substrate mounted with the foreign particle deposition preventive film can prevent deposition of additional foreign particles thereto. The foreign particle deposition preventive film is spaced apart from the surface of the substrate and therefore, even in the presence of relatively small foreign particles on the foreign particle deposition preventive film, an image of the foreign particles will not be transcribed upon the wafer. Accordingly, when the foreign particle deposition preventive film is used, the process covering cleaning of the substrate and exposure of the wafer is carried out as follows.
A substrate is first cleaned and the presence or absence of foreign particles such as dust on a pattern surface and a non-pattern surface of the substrate is examined. If no foreign particle such as dust is found, a foreign particle deposition preventive film is mounted to the substrate by using a suitable tool to cover the entire substrate surface inclusive of the pattern surface. The substrate with the foreign particle deposition preventive film, hereinafter referred to as a substrate assembly, is also checked for the presence or absence of foreign particles on the surface of the assembly according to a method described in Japanese Patent Unexamined Publication No. 59-82727. If no foreign particle is found, the substrate assembly is set in a cassette and the cassette is fed to a reduction exposure apparatus, followed by an exposure process.
Thanks to the foreign particle deposition preventive film, a foreign particle of less than 20 to 30 .mu.m in size deposited on the surface of the pellicle or film can be considered to be negligible and yield of production of chips can be improved.
However, since probability of foreign particle deposition is in reverse proportion to squared size of a foreign particle, probability of deposition of a foreign particle being of more than 20 to 30 .mu.m size can not be neglected completely. Further, in the exposure apparatus, there is pressumably a great possibility that a foreign particle is deposited to the foreign particle deposition preventive film in the course that the substrate assembly, typically stored in a magazine, is picked up and then set at a mask table. For these reasons, in order to aim at further improved yield, it is necessary to make an examination of the presence or absence of a relatively large foreign particle on the foreign particle deposition preventive film of the substrate assembly before exposure.